Inverters used in three-phase balanced systems typically have three inverter legs for modulating output voltages or currents. In particular, to achieve high bandwidth control of ac current, voltage source inverters with fast internal current regulators are often used. For such inverters, load inductances (including stray inductances of motors, transformers and output filter inductors) are part of the active power electronic circuit which functions as a current source. Advantageously, with fast internal current regulation, it is possible to avoid undesirable transients or oscillations due to the load inductances in conjunction with the back electromotive force (emf) or the capacitive filter of the inverter load. Furthermore, fast current regulation allows for instantaneous torque response in motor drives such as those for induction motors, synchronous machines, switched reluctance motors and permanent magnet machines.
Current control strategies for three-phase balanced systems which minimize current ripple, harmonic content, switching frequency and dc bus current ripple are well-known. Unfortunately, however, most three-phase power supplies and voltage-source, current-fed generator systems supply unbalanced loads. A neutral current is required in systems having unbalanced loads in order to maintain a set of three balanced voltages. To this end, a fourth inverter leg is commonly added to control the neutral current. One type of current regulator used for four-legged inverters is the well-known bang-bang controller. Disadvantageously, however, in systems using bang-bang controllers, the total harmonic distortion (THD) and dc bus current ripple are often too high. Other types of current regulators used for four-legged inverters include predictive controllers which select the next most optimal switching state of the inverter by determining all the possible future current trajectories of the system. Such predictive controllers, however, typically require complex digital signal processors.
Accordingly, it is desirable to provide a relatively simple current regulator for a four-legged inverter which maintains high bandwidth current control under unbalanced conditions and furthermore minimizes current ripple, harmonic content and current stresses on the switching devices, while substantially fully utilizing the dc bus voltage.